The present invention relates to the dyeing of fibrous articles containing wool with anionic dyes.
Anionic dyes such as acid dyes and pre-metallized dyes are widely used for the dyeing of wool. In conventional dyeing processes using such dyes, articles containing wool are immersed in an aqueous bath containing a solution of the dye after any pre-treatment processes such as scouring. While a wide variety of dyeing equipment is used, it is typical for all of the dye to be used in the process to be present in the bath initially. The bath containing the dye and the article to be dyed is also usually at a low initial temperature, e.g., 80.degree.-120.degree. F. (26.7.degree.-48.9.degree. C.) and the temperature is increased gradually to an elevated temperature often as high as the boiling point as the dyeing progresses. "Metallizing" treatments with, e.g., with potassium dichromate, are often performed subsequently to dyeing to increase dye light and/or wash fastness.
While high quality dyeing can be achieved using the conventional dyeing process for some acid dyes such as small molecule "levelling" dyes, dye cycles to achieve levelling with such anionic dyes are sometimes extremely long and are therefore costly. In addition, long dye cycles are undesirable since the maintenance of the bath at a high dyeing temperature for a long period of time can decrease the strength of wool. It is very common to dye wool using large molecule acid and pre-metallized dyes which are desirable for applications requiring good light and/or wash fastness. However, undesirable dyeings can result with conventional dyeing processes using large molecule acid and pre-metallized dyes since individual wool fibers may not dye uniformly. Often, one end of a fiber will dye readily while the other end will absorb little dye resulting in a "skittery" dyeing in the article, i.e., noticeable localized dark/light areas.
Large molecule acid and pre-metallized dyes are often referred to as "structure sensitive" dyes since non-uniform dyeing can result from minor variations in the fiber physical structure. While dye-levelling and/or retarding agents can be added to the dye bath to improve dyeing uniformity with structure sensitive dyes, such agents sometimes provide only limited increases in dye uniformity and usually have disadvantages including increased initial expense and higher cost to treat the spent dyeing bath. In addition, because of their retarding effect, such chemical agents can sometimes increase dyeing cycles or make it difficult to obtain deep colors or dark shades. Also, dye yields from anionic dyes, i.e., the strength of color produced from a given quantity of dye on the fiber, are sometimes not as high as desired.